Soils & Grow Media

The material that your plants grow in is called the “growing medium”. Dozens of different ingredients are used in varying combinations to create homemade or commercial growing media. By understanding the functions of growing media, you can evaluate the qualities of individual types and select which ones might work best for your container vegetable garden. The choice is very important because your plants are dependent on a relatively small volume of growing medium. Unlike their cousins growing in garden soil, containerized plant roots cannot grow around obstacles or mine the soil far and wide for nutrients and water.

Growing medium has three main functions- 1) supply roots with nutrients, air, and water, 2) allow for maximum root growth, and 3) physically support the plant.

Roots grow in the spaces between individual particles of soil. Air and water also travel through these pore spaces. Water is the medium that carries nutrients that plants need to fuel their growth, and air is needed for root growth and the health of soil microorganisms that help supply plants with nutrients.

Irrigation water moves through the pore spaces, pushing out the air. If excess water cannot drain away, fresh air cannot enter and roots will suffocate.

Select light and fluffy growing media for good aeration and root growth.

Qualities of Different Types of Growing Media

Garden Soil— never use this by itself for container gardens. Soils hold water and nutrients very well and can drown roots growing in a container. Diseases and weed seeds can also be a problem. And soil is heavy which is an advantage if you are trying to anchor top-heavy plants and pots, but a disadvantage if you want to move pots.

Commercial Soil-Less Mixes— these are an excellent choice for containers. They are lightweight, drain well, hold water and nutrients, and are generally free of weeds, insects, and diseases. They have a pH of about 6.2 and are typically comprised of sphagnum peat moss, perlite, vermiculite and small amounts of lime and fertilizer. Examples of soil-less mixes are ProMix™, ReddiEarth™, Jiffy Mix™, and Sunshine Mix™. (To produce “organic” soil-less mixes, suppliers omit chemical wetting agents and substitute organic for chemical fertilizers.)

Other Types of Commercial Mixes— are advertised as “top soil”, “planting soil”, “planting mix”, or “potting soil”. They vary a great deal in composition and quality. Avoid mixes that contain sedge peat, feel heavy or gritty, have very fine particles, or appear clumped.

Sharp Sand— use only coarse builder sand, not play sand. Sand increases porosity because of the large particles. It is relatively inexpensive and heavy.

Bark Fines and Wood Mulch— these are high in carbon and low in nutrients and not recommended for container vegetables.